1. Field of the Invention
The present invention relates to a method for producing a synthetic silica glass suitable to be used for optical systems which use a light of a specific wavelength region of 400 nm or shorter, preferably 300 nm or shorter in photolithography, especially ultraviolet lithography technique.
Furthermore, the present invention relates to a synthetic silica glass produced by the above method and optical members such as lenses and mirrors comprising the silica glass.
2. Related Background Art
Recently, VLSI is being further increased in integration density and enhanced in function, and realization of "system-on-chip" comprising a chip implemented with a larger system is increasingly developed in the field of logic VLSI. As a result, fine processing and increase of integration density on wafers such as silicon as a substrate are required. An exposure apparatus called a stepper is used in the photolithography technique in which a fine pattern of integrated circuit is exposed and transferred onto a wafer such as silicon.
For example, taking DRAM as an example among VLSIs, when LSI is developed to VLSI and the capacity is increased in such a way as 1K.fwdarw.256K.fwdarw.1M.fwdarw.4M.fwdarw.16M, the finer stepper of 10 .mu.m.fwdarw.2 .mu.m.fwdarw.1 .mu.m.fwdarw.0.8 .mu.m.fwdarw.0.5 .mu.m in width of processed lines is needed.
Therefore, lenses in projection optical system of stepper are required to have a high resolution and a large focal depth. The resolution and focal depth determine depending on wavelength of light used for exposure and N.A. (numerical aperture) of lens.
The angle of diffracted light increases for the finer pattern and unless N.A. of lens is large, the diffracted light cannot be taken in. Further, with the wavelength .lambda. of exposure light being shorter, the angle of diffracted light in the same pattern decreases and, thus, N.A. may be small.
Resolution and focal depth are shown by the following formulas. EQU Resolution=k1.multidot..lambda./N.A. EQU Focal depth=k2.multidot..lambda./N.A..sup.2
(wherein k1 and k2 are constants of proportionality)
Accordingly, in order to improve the resolution, N.A. is increased or .lambda. is shortened, and, as is clear from the above formula, it is advantageous in respect of the depth to shorten the .lambda.. From this viewpoint, the wavelength of light source is gradually shortened from g-line (436 nm) to i-line (365 nm) and further to KrF excimer laser (248 nm) or ArF excimer laser (193 nm).
Moreover, the optical system mounted on the stepper comprises combination of optical members such as many lenses and even if decrement of transmittance per one lens is small, it adds up to the total decrement of all the lenses to lead to decrease of luminous power at the irradiated face, and, thus, increase of transmittance is desired for optical members.
Therefore, particularly, in an optical system which uses a light of wavelength region shorter than 400 nm, an optical glass produced by a special method which takes into consideration the decrease of transmittance caused by the combination of shortening of wavelength with optical members is used. Furthermore, in an optical system which uses a light of wavelength region shorter than 300 nm, it has been proposed to use synthetic silica glass or fluoride single crystals such as CaF.sub.2 (fluorite).
On the other hand, optical members high in uniformity of refractive index (small in distribution of refractive index in the measured region) are needed as lenses of projection optical systems to realize fine line width and obtain fine and clear exposed and transferred patterns. However, owing to the recent increase of the exposed area with enlargement of wafer size of semiconductors, aperture or thickness of these optical members expands and it becomes further difficult to obtain the required quality.
In order to realize high transmittance for ultraviolet light, it is necessary to reduce the concentration of impurities in silica glass. As a method for producing such silica glass, the flame hydrolysis method is generally employed which comprises jetting from a burner an Si compound gas as a starting material gas, a carrier gas carrying the Si compound gas (e.g., H.sub.2 gas or O.sub.2 gas) and a combustion gas used for heating and depositing silica glass powder on a target in the flame.
It is known that according to this method, since the concentration of impurities in the starting material gas and the combustion gas can be easily held down, a high purity silica glass can be obtained, but the silica glass to be used as optical members for ultraviolet lithography is also required to have a uniform refractive index distribution. The main causes for the refractive index distribution of the silica glass becoming ununiform are fluctuations of various conditions occurring in synthesis of the silica glass, such as change of temperature distribution at the synthesis face due to the flame, flame hydrolysis reaction or thermal decomposition and thermal oxidation reactions, change of diffusion state of impurities into the glass, and the like.
Under the circumstances, Japanese Patent Application Laid-Open Gazette No. Hei. 6-234531 (234531/94) discloses a method for making uniform the refractive index distribution of silica glass. According to this method, a temperature distribution at which the uniformity of the refractive index is optimized is formed at a head portion of an ingot by relatively moving the burner and the ingot in accordance with the temperature distribution at the head portion, thereby to attain the uniform refractive index distribution.
However, when the synthetic silica glass obtained by such a conventional method is used, there is a limit in improvement of resolution and this is still not satisfactory.